Download Crossing Over - Biology D118

Crossing Over
1.
The process of crossing over may cause
permanent changes in the genetic makeup of
chromosomes. Crossing over occurs during
prophase 1 of meiosis w and takes place between
paired HOMOLOGOUS chromosomes.
2.
We will follow a homologous pair of
chromosomes and see what happens when
crossing over occurs and when it fails to occur.
We will point out that only two types of gametes
are possible without crossing over, but FOUR
different types of gametes are possible when
crossing over does occur, making it an essential
source of variability.
3.
In the first process, variability is not
introduces into a species because crossing over
does not take place. We begin by looking at the
pair of chromosomes in diagram 1, at the top.
This is a homologous pair, meaning that DNA has
duplicated just before the start of meiosis. The
result of this DNA duplication was a pair of
sister
chromatids for
each homologous
chromosome. The first homologous chromosome
now consists of sister chromatids A (A) and
Sister chromatids B (B).
4.
As we see in the first diagram. Sister
chromatids A had gene 1 (C) and gene 2 (D). Both
sister chromosomes have these genes because
they are essentially copies of one another.
Sister chromatids B have gene 3 (E) and gene 4
(F). Genes 1 and 3 may be alleles of the same
gene, while genes 2 and 4 may also be alleles of a
gene, but we are not concerned with dominance
or recessiveness here. In diagram 2, the sister
chromatids appear as they did in diagram 1.
Diagram 2 represents the point in prophase 1 at
which the homologous chromosomes come
together and sister chromatids form a tetrad.
The chromatids stand side by side, crossing over
has not occurred yet.
5. In diagram 3 the chromatids have separated
during anaphase 1 and you can see four
chromosomes (G). Now, in telophase, we note
that the first chromosome contains genes 1 and
2 and the third and fourth chromosomes have
genes 3 and 4. As you can see there are two
types of chromosomes which means that only
two types of sex cells are possible.
6.
We now consider the second part of the
diagram, where crossing over takes place. Once
again, we focus on sister chromatids A (A) and B
(B). This diagram is similar to the one above it
but, as we move to diagram 2 we see a
difference. Here sister chromatids exist in a
tetrad formation, but crossing over occurs.
Notice that the second and third chromatids
cross one another, and exchange of genes is
taking place.
7.
As we move to diagram 3, we see the
effects of crossing over. Chromatids 2 in sister
chromatids A now has a gene 3 (E) and its
original gene 2 (D). In sister chromatids (B), the
third chromatid now has gene 1 (C) PLUS its
original gene 4 (F). Thus, the second and third
chromatids are different because of crossing
over. Only the first and fourth chromosomes
remain the unchanged.
8.
Now we move to the fourth diagram, in
which the chromatids have separated during
anaphase. Now they exist as chromosomes. When
we examine the genetic composition of the
chromosomes, we see a dramatic difference
because of the crossing over. Chromosome 1 has
gene 1 and 2 (D) as expected, but chromosome 2
has genes 3 (E) and gene 4 (F). They are now
four different chromosomes, and when these
chromosomes are distributed to sperm or egg
cells, four different cells can result. Without
crossing over only two different cells could
result.
9. Crossing over is tremendously significant in
the evolutionary process. For example, a
chromosome might acquire an advantageous
allele that joins an already advantageous allele.
At fertilization, the offspring might receive this
allelic pair and have a genetic advantage. Thus,
the offspring would be favored by natural
selection to reach reproductive age and pass the
advantageous alleles to it s offspring. This is one
of the ways in which crossing over contributes
to evolution.
Crossing Over
Name____________________________________
Questions
1. When does crossing over occur?
2. If crossing over does not occur, how many different types of chromosomes are
possible?
3. What actually occurs when crossing over takes place (HINT – look at picture
#2 in the lower diagram).
4. How many different types of chromosomes develop when crossing over occurs?
5. What is the advantage?